The present invention relates to thrust-producing rotor systems for model helicopters. More particularly, the present invention relates to main rotor systems for model helicopters that can withstand repeated crashes of the model helicopter without significant degradation of their flying qualities.
In general, helicopters are flying machines with the ability to hover and fly forwards, backwards, and sideways. With all of their spinning mechanisms and mechanical linkages, helicopters are intrinsically interesting. It is little wonder that aviation buffs have always taken special interest in model helicopters.
The first practical radio-controlled model helicopters flew in about 1969. Since then, designers have endeavored to develop model helicopters that fly better and cost less. Model helicopter designers, who are more often hobbyists than professional engineers, frequently fail to consider the differences between large-scale and small-scale structures and aerodynamics and base their model designs on full-size helicopters.
When full-size helicopters are scaled down to model proportions, however, their small main rotor systems are typically so inefficient at producing lift that many small model helicopters can barely get off the ground. To compensate for low lift main rotor systems, the structures of small helicopters are typically light weight and fragile and incapable of absorbing much abuse before breaking. As a consequence, conventional model helicopters are expensive, complex, and fragile and the general public consensus is that conventional small model helicopters are impractical and undesirable.
The main rotor blades on conventional radio-controlled model helicopters are usually made of either laminated wood, styrofoam, or fiberglass. Rotor blades for radio-controlled models should not be confused with the rotor blades of toys or free-flight models. Rotor blades on toy helicopters have modest operational requirements and are not feasible for use on larger, heavier, more powerful models that must carry a heavy radio control system.
Most rotor blades on radio-controlled model helicopters are made of laminated wood and covered with a thin plastic film. Airfoil selection and "planform" shape of these blades are limited by the capabilities of the wood-working tools used in their manufacture. Wooden rotor blades typically are made from rectangular blanks that are drawn through revolving cutters which form the upper and lower surfaces of the blades. For this reason, they usually are built straight (with no twist or taper) and have a single airfoil section along their entire length. Wooden rotor blades are relatively inexpensive, but require time-consuming hand assembly, finishing, and balancing. Wooden rotor blades often break when the model helicopter crashes.
Styrofoam rotor blades usually have a rigid structural core covered with an airfoiled skin made of hard polystyrene or urethane foam. The core, which may be made of metal, wood, or glass fibers is designed to withstand high radial flight loads. The styrofoam skin surrounding the core forms the aerodynamic shape of the blade and may be molded with complicated geometries. Styrofoam blades are moderately expensive and very susceptible to crash damage since the styrofoam skin is very soft. Even minor impacts with objects or the ground can render them completely unusable.
Fiberglass rotor blades are usually formed in molds by hand, are hollow or have a styrofoam core, and may be designed with complicated geometries. Fiberglass blades are extremely stiff both torsionally and longitudinally. Hand-built fiberglass rotor blades are very expensive and may be damaged or destroyed in a crash.
In contrast to the wood, foam, and fiberglass rotor blades of existing radio-controlled model helicopters, the main rotor blades of the current invention are preferably made from a solid molded plastics material such as nylon.
Solid plastic blades have always had several serious drawbacks. Rotor blades made of stiff, low weight, plastic materials, such as injection-molded polystyrene, tend to shatter upon impact with the ground. Rotor blades made of materials such as flexible polyurethane are highly impact resistant, but cannot maintain their shape when subjected to high centrifugal and aerodynamic flight loads which leads to such problems as blade flutter.
Common injection-molded plastic materials are also 2 to 5 times higher in density than the maple and obechi woods commonly used in rotor blades for radio-controlled model helicopters. If plastic rotor blades are geometrically identical to wooden blades, they will not improve flight performance but they will weigh substantially more. In addition, the high weight of these plastic rotor blades can radically increase the centrifugal force a rotor blade must withstand in flight. A rotor blade made of wood generating a nominal centrifugal force of 500 pounds (as is common on large radio-controlled model helicopters) would generate a force of 1,000 pounds to 2,500 pounds if made of solid plastic. In this case, the structure of the main rotor hub would have to support an additional 500 to 1500 pounds of force with no gain in flight performance.
As a result of all these drawbacks, model helicopter designers have viewed molded plastic materials as inappropriate for rotor blades. They are accustomed to the idea that traditional rotor blades will break during a crash and treat the rotor blades as a replaceable item.
Although many rotor designs exist, no known design or method of manufacture has produced a main rotor system that is capable of efficiently lifting a radio-controlled model helicopter into the air and surviving repeated energetic crashes, such as impacts with a brick wall or tree trunk. What is needed are efficient, durable, and inexpensive rotor elements for use on model helicopters. To be practical, the rotor elements must generate enough lift to allow the helicopter to fly. To be popular and appropriate for the general public, the rotor elements must absorb the punishment of the unsophisticated novice. To be a commercial success, the rotor elements must be inexpensive and easy to manufacture.
What is needed is a main rotor system that efficiently lifts a radio-controlled model helicopter into the air and is capable of surviving repeated crashes. Such a main rotor system would be welcomed by model helicopter enthusiasts.
In accordance with the present invention, a main rotor is provided for use on a model helicopter. The main rotor includes a main rotor shaft rotatable about a main rotor rotation axis and first and second main rotor blades linked to the main rotor shaft to extend radially outward from and substantially perpendicular to the main rotor shaft. The first and second main rotor blades rotate with the main rotor shaft in a steady-state main rotor blade plane of rotation when the main rotor blade plane of rotation is perpendicular to the main rotor rotation axis. The main rotor further includes a mechanism to pitch the first and second main rotor blades about first and second main rotor blade pitching axes, respectively. Each of the first and second main rotor blades include a blade root linked to the main rotor shaft, a blade tip spaced apart from the blade root, a leading edge, a trailing edge spaced apart from the leading edge, a plurality of chord lines extending in a straight line between the leading edge and the trailing edge perpendicular to the first and second main rotor blade pitching axes, an inboard section situated adjacent to the blade root, and an outboard section situated adjacent to the blade tip.
The inboard section includes a first steady-state angle-of-attack defined as an included angle between one of the plurality of chord lines in the inboard section and the main rotor blade plane of rotation. The outboard section includes a second steady-state angle-of-attack defined as an included angle between one of the plurality of chord lines in the outboard section and the main rotor blade plane of rotation. The first steady-state angle-of-attack is greater than the second steady-state angle-of-attack. One of the plurality of chord lines in the inboard section includes a first length and one of the plurality of chord lines in the outboard section includes a second length that is shorter than the first length. The inboard section includes a cupped-shaped cross-section between the trailing edge and the leading edge and the outboard section includes a flat cross-section between the trailing edge and the leading edge.
In a preferred embodiment of the present invention, the inboard and outboard sections of the main rotor blade include airfoils having a chord length, camber, steady-state angle-of-attack, and thickness within a specified range. These airfoils provide a relatively lightweight rotor blade that produces a desired amount of lift for the model helicopter.
A main rotor blade in accordance with the invention may be bent between a nominal position and a plurality of bent positions. Each main rotor blade includes a blade root linked to the main rotor shaft, a blade tip spaced apart from the blade root, and a blade body that may be bent so that the blade tip touches the blade root. After the blade body is bent to one of its bent positions and released, the blade body will return to its nominal position. The ability of the main rotor blades to bend and return to their nominal position provides rotor blades that are less likely to break in a crash of the model helicopter.
In preferred embodiments of the present invention, the main rotor system includes stabilizer rotor blades with airfoils pitched to a steady-state positive angle-of-attack. Because the stabilizer rotor blades are at a positive angle-of-attack, the stabilizer rotor blades produce lift to assist the main rotor blades.
The main rotor system further includes stabilizer rotor blade extensions that connect the stabilizer rotor blades to the main rotor hub and a separate stabilizer pivot rod arranged to extend along a stabilizer rotor blade pivot axis through the stabilizer rotor blades. The stabilizer rotor blade pivot extension is spaced apart from the stabilizer pivot rod so that the stabilizer pivot rod does not have to support radial loads generated by the stabilizer rotor blades.
A system is provided for changing the steady-state pitch angle of a main rotor blade. The system includes interchangeable main rotor element sets such as interchangeable rotor blade grips. To change the steady-state pitch angle of the main rotor blade, a first interchangeable blade grip that orients the main rotor blade in a particular steady-state pitch angle is replaced with a second interchangeable blade grip that orients the main rotor blade in a different steady-state pitch angle.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.